Today's healthcare practitioners are usually provided with ready-to-use medical devices. This is particularly true of syringes that are used to administer parenteral drugs and other medical solutions, because these devices can be sterilized during production. A syringe typically includes a barrel that has a substantially closed distal end, and an opposite proximal open end that is sealed by a slidable piston plunger. The distal end of the syringe includes a dispensing port, typically communicating with a luer fitment, for dispensing the contents of the syringe. A removable end-cover, such as a luer cap, is placed over the luer fitment during manufacture to seal contents within the syringe barrel. The syringe may be prefilled with a liquid, part-filled with a solid or liquid, or empty. Prefilled syringes are advantageous in avoiding confusion regarding whether a vial is multi-dose or single-dose, or which diluent should be used with a given medicament; and prefilled syringes may provide a suitable means for storage and shipment of sensitive pharmaceuticals such as biologics. Additionally, use of prefilled syringes, particularly those with safety mechanisms, may limit a health professional's risk of inadvertent needle-stick injuries or possible exposure to infective pathogens or other contaminants associated with used syringes.
Further regarding pathogens and contaminants, the practice of sharing used needles or syringes between successive users can transfer pathogens such as Human Immunodeficiency Virus and Hepatitis Virus, with subsequent severe repercussions for the infected and high social cost in providing medical care for sufferers. In response to this practice, safety syringes have been developed that prevent re-use of a used syringe.
For example, one approach has been to develop safety syringes comprising a compressed spring mechanism that, following injection, permanently retracts the needle into the barrel of the syringe. Generally, spring decompression is relatively uncontrolled, and excessively forceful needle retraction can result in blood splattering when air is forced from the syringe barrel as the needle retracts into the barrel. Further, many such retraction syringes have highly specialized needle assemblies that stymy replacing a defective needle or selecting a desired needle size. An additional complication involves ensuring that the needle does not inadvertently retract into the syringe barrel, for example when piercing the skin before injection, while ensuring retraction after delivery of syringe contents. Mechanisms that achieve these functions may challenge designers, particularly if competing interests of manufacturing costs and reliability are balanced for a commercially viable retraction syringe. These problems are often more marked in the context of a retraction syringe having a replaceable needle assembly. Therefore, there is a need for more “user friendly” retractable syringes that do not compromise the safety features provided by the syringe.